In the conventional construction of coated catalysts, a cylindrical catalyst substrate (normally either circular or elliptical in cross-section) having a circumferential periphery and an interior honeycomb structure is coated with precious metal. The substrate is first subjected to an alumina wash coat, fired in an oven, and then passed under a precious metal liquid solution or suspension shower. Conventionally the shower is an aqueous precious metal solution shower. The aqueous metal solution coats the entire substrate, including the interior honeycomb structure and the circumferential periphery. The substrate is then fired to produce the final coated catalyst.
While the coated catalyst produced according to conventional techniques perform their intended functions very well, they are unnecessarily expensive since the precious metal coating on the circumferential periphery of the substrate is unnecessary (no gaseous components to be catalyzed are ever brought into contact therewith during normal use of the catalyst substrate during normal use). The circumferential periphery also has a tendency to collect a thicker coating thereon than the interior honeycomb structure, resulting in even more wastage of platinum, palladium, rhodium and like precious metals. This is especially important since when the catalyst substrates are utilized as the catalytic components of catalytic converters for motor vehicles they are (under present procedures) disposed of, resulting in a net loss of precious metal.
In order to prevent the waste of precious metal in this manner, there have been various proposals for significantly reducing the precious metal coating on the cylindrical catalyst substrate circumferential periphery. One proposal has been to utilize cellulose based Cellon shrink seal material. A sheet of Cellon is applied wet in an expanded form as an exterior wrap for the substrate, with an intermediate area wrapped with duct tape. The wrapped substrate is then carefully dried with convected air at a temperature of about 110.degree. C., and the substrate is passed through the precious metal containing liquid shower. The wrap is then removed prior to firing of the substrate, and discarded. While such a proposal is effective in reducing the amount of coating of the circumferential periphery of the substrate by platinum, palladium, and the like, the technique is not readily practiced, is not suited to conventional production methods, and reduces metal concentration in several peripheral cells of the honeycomb structure. Additionally, the accessory drying step must be practiced prior to passing the substrate under the precious metal containing liquid shower.
Another prior proposal for reducing the concentration of precious metal on the substrate circumferential periphery is to spray a silicone solution onto the substrate, and then dry the substrate at about 80.degree. C. for about an hour. The silicone spray is effective in reducing precious metal concentrations on the substrate circumferential periphery, however the extra costs associated with the silicone spraying and related necessary steps may be substantial enough to significantly offset any savings resulting from minimizing precious metal wastage.
According to the present invention a method (and device for implementation of the method) is provided which allows significant savings in precious metal usage during the production of coated catalysts in a simple and easy manner, and without significantly increasing the costs of producing the coated substrate and without introducing significant time delays into the processing sequence.
According to the present invention, a method of producing a coated catalyst from a cylindrical catalyst substrate having a circumferential periphery and an interior honeycomb structure is provided utilizing a jacket of liquid impermeable material having first and second ends, fastening means for attaching the first and second ends together and having a height substantially the same as the height of the cylindrical catalyst substrate. The fastening means preferably are hook-and-pile fastening components commonly sold under the name VELCRO.
A method according to the present invention comprises the following steps: (a) Wrapping the jacket around the cylindrical catalyst substrate circumferential periphery and fastening the jacket ends together, to provide a cylindrical catalyst substrate having the circumferential periphery thereof covered so that a precious metal liquid solution or suspension will not coat it. (b) Passing the wrapped cylindrical catalyst substrate through a shower of precious metal liquid solution or suspension so that the honeycomb structure surfaces are coated with precious metal. (c) Removing the jacket by releasing the fastening means attaching the ends thereof together so as to provide a cylindrical catalyst substrate having a precious metal coated interior honeycomb structure and a circumferential periphery uncoated by precious metal.
The method also preferably comprises the further step of (d) reusing the removed jacket to wrap other cylindrical catalyst substrates. Additionally, the wrapping of the jacket around the substrate consists of the only substep for covering the circumferential periphery of the substrate, no drying or other time-delay introducing steps being necessary. Between steps (c) and (d), the jacket may be rinsed off with a rinse liquid, the used rinse liquid collected, and the collected rinse liquid used as a component of the shower of step (b).
A jacket particularly adapted for wrapping a cylinder in practicing the method of the present invention is also provided according to the present invention. The jacket comprises a first rectangular flexible sheet comprising a liquid-impermeable plastic film or the like, such as a polyvinyl or polyethylene film. A second rectangular flexible sheet is also provided comprising a liquid resistant fabric, such as a fabric of nylon or Dacron. The first and second sheets have substantially the same length and width, and means are provided for attaching the sheets together along at least the edges thereof to provide a liquid impermeable jacket. A pile fastening component is attached to the jacket along the width thereof at a first end thereof, and a hook fastening component, compatible with the pile fastening component, is attached to the jacket along the width thereof at a second end thereof. If necessary or desirable a piece of resilient material may be operatively disposed along the length of the jacket at one edge thereof for providing enhanced sealing when wrapped around the cylindrical catalyst substrate, and the resilient material may be held in place by overlapping portions of the second sheet stitched together with the body of the second sheet and the first sheet. Alternatively, the jacket can be a single sheet of fiber-reinforced water impermeable plastic film.
It is the primary object of the present invention to simply and economically effect reduction of the amount of precious metal on the circumferential periphery of a coated catalyst formed from a cylindrical catalyst substrate. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.